1) Field of the Invention
The present invention relates to an objective lens, a combination of objective lenses, and a method for adjusting an optical system using an objective lens.
2) Description of Related Art
In the case where none of lens elements constituting an optical system is decentered from a single axis, that is, in the case of an ideal optical system, a position at which aberrations are smallest exists on the axis. Also, in the case of the ideal optical system, the axial position coincides with the field center. Therefore, in the ideal optical system, observation and measurement of a minute specimen would better be performed at the field center.
In an actual optical system, however, each lens often is decentered subtletly or slightly. This is because the lenses and lens frames involve tolerance. As a result, in the actual optical system, perfect coincidence of the axial position with the field center seldom occurs.
Therefore, in the actual optical system, adjustment is made, during or after assemblage of the optical system, to minimize decentering of individual lenses. For example, one adjustment method in assembling an objective lens is to use a pinhole specimen. According to this method, adjustment is made so that images of pinholes have substantially the same shape over the entire field. Alternatively, upon the pinhole specimen being shifted back or forth in reference to the in-focus position to cause defocus condition, adjustment is made so that image blur of the pinholes is symmetrically formed.
However, according to the above-described adjustment method, the amount or the distribution pattern of aberrations is not measured. Therefore, even if aberrations of the entire lens system can be minimized, it is impossible to locate, in the field, a point at which aberrations are smallest. As a result, if a plurality of objective lenses of the same type and magnification are individually adjusted according to the above adjustment method, it would cause problems such that image appearance or measured values vary by individual objective lens.
Also, in the case of an optical system that is configured to perform observation or measurement via an imaging lens or a relay lens in addition to the objective lens, even if the axial position and the field center coincide in the objective lens, decentering of the imaging lens or the relay lens would shift the axial position and the field center from each other. As a result, in the optical system as a whole, the axial position and the field center do not coincide. To be worse, since the axial point cannot be located, it is impossible to make the axial position and the field center to coincide.
An object of the present invention is to provide an objective lens that allows a user to locate the axial point of an optical system in the field.
Another object of the present invention is to provide an objective lens that can adjust a predetermined optical system so that positions in the field at which a predetermined aberration condition is satisfied substantially coincide in the optical system, and a method for adjusting an optical system using this objective lens.
Still another object of the present invention is to provide a combination of objective lenses used for observation or measurement, the combination allowing a user to locate a point in the field at which a predetermined aberration condition is satisfied.
In order to attain the objects set forth above, an objective lens according to the present invention includes a lens system and an index member provided with an index that indicates a reference position, wherein the index member is disposed at the front-side focal position of the lens system or in the vicinity thereof.
Also, a combination of objective lenses according to the present invention at least includes a first objective lens and a second objective lens, wherein the first objective lens is composed of a lens system alone and the second objective lens has a lens system and an index member provided with an index that indicates a reference position. The index member is disposed on the front-side focal position of the lens system of the second objective lens or in the vicinity thereof. The first objective lens is adjusted so that aberrations at a position corresponding to the position of the index that indicates a reference position of the second objective lens satisfy a predetermined condition.
Also, according to the present invention, a method for adjusting an optical system using an objective lens includes a step of mounting on an apparatus provided with an image sensor an objective lens that has a lens system and an index member disposed at the front-side focal position of the lens system or in the vicinity thereof, the index member being provided with an index that indicates a reference position, and capturing an image of the index that indicates a reference position with the image sensor, a step of calculating the position of the index that indicates a reference position in the captured image and storing the position of the index, a step of mounting a predetermined optical system in place of the objective lens on the apparatus having the image sensor and measuring aberrations of the predetermined optical system, and a step of adjusting the optical system so that aberrations at a position that corresponds to the stored position satisfy a predetermined condition.
According to the present invention, in constructing or using an optical system that is required to have a measurement precision of the order of nanometers, a user can easily determine a certain point on the specimen surface that is functionally characterized in the optical system.
This and other objects as well as features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.